This pre-lab will present the general concepts related to some of the techniques used in the fields of microbiology and molecular biology and how these techniques relate to understanding the relationship between DNA, genes, and traits expressed by an organism.
General Purpose
Procedural
Bacterial cultures, like Escherichia coli, can be grown in several ways. One method that is commonly used is to grow the bacteria in a Petri dish on an agar based growth medium. This method of growing bacteria can allow the bacteria to be exposed to different growth conditions. This can be useful if the effects of different treatments on the bacterial growth or survival need to be assessed. In order to determine the impact a treatment may have on a bacterial population, the number of bacteria present in the Petri dish must be small enough to count (between 20–300 per Petri dish). Since bacterial organisms are very small it is impractical to try to count them as individual cells. However, each of the cells will reproduce and eventually the individual cells will grow into colonies of cells that can be counted.
To facilitate the growth of individual colonies of bacteria, the cells that are used to inoculate the agar growth medium in the Petri dish must be: 1) dilute enough to give appropriate numbers and 2) spread evenly over the surface of the agar growth medium. The technique used to achieve these goals is the spread plate method. In this technique a small volume of dilute bacterial liquid culture is pipetted onto the surface of the agar growth medium. The liquid is then spread across the surface of the medium using a cell spreader. After allowing the liquid a short time to be absorbed into the growth medium the Petri dishes can then be subjected to treatments and/or incubated for colony growth.
All of the procedures must be done using sterile technique to avoid contamination of the Petri dish with unwanted organisms.
Below is a demonstration of how to make a spread plate using sterile technique.